Composite films containing acrylic based polymers and urethane polymers are known as films that can have both high strength and high breaking extention. In JP 2001-520127 W, a multi-layered film is disclosed which comprises an interpenetrating polymer network layer (IPN layer) and at least one fluoro-containing polymer layer used as a film for protecting surfaces of automobiles and the like. An IPN composite material containing a urethane polymer and an acrylic polymer is used for forming the IPN layer in the multi-layered film. The IPN layer is prepared by applying a mixture of an acrylic monomer, an acrylic crosslinking agent, and a crosslinked urethane precursor obtained by using a polyol and a polyisocyanate, on a substrate and by polymerizing and crosslinking the acrylic monomer and the polyol/polyisocyanate which is a urethane precursor, with heat in an incoherent manner.
According to the method, there is an advantage of not likely to occurring limitations (restrictions) due to the kind, combination, blending ratio, and the like of the monomers used therein. However, urethane polymerization is a polyaddition reaction, which is slow in comparison with a chain reaction of, for example acryl, so that there has been a problem in terms of productivity.
In order to solve such a problem of productivity, when an IPN layer is obtained by utilizing successive reaction and photopolymerization as disclosed in JP 2003-96140 A, the crosslinked urethane polymer becomes in a swollen state in the presence of the acrylic monomer and the crosslinking agent, so that such a problem has occurred that the viscosity of the syrup is remarkably increased and the application on the substrate by coating or casting has become extremely difficult.
In addition, dusts and pebbles sometimes hit on a painted surface of the body while driving an automobile or the like, and in particular when driving on a road surface in a poor roadway condition or a road surface having rock salt, sand and gravels spread thereon in aid of snow-removal in cold regions, the painted surface is easily damaged and there is a problem of generating rust from the damaged portion on the painted surface. In the automobile industries, a transparent adhesive tape is sometimes applied to prevent from causing damage to painted surfaces of automobile bodies. Polyurethane substrates are used as a substrate for such a transparent adhesive tape.
However, polyurethane is known to create a colored substance having a conjugated structure or a nitrogen-containing colored substance due to a photoreaction. Accordingly, a film containing polyurethane is initially colorless and transparent, but the film after being remained outdoors shows yellowing by being exposed to ultraviolet rays and also the surface sheen may disappear and the good appearance may be decreased.
Although an ultraviolet absorber can be added in order to prevent yellowing, when a film is formed by carrying out a photopolymerization reaction by using a photopolymerization initiator, functions of the photopolymerization initiator is not sometimes exhibited sufficiently due to the coexistence of the ultraviolet absorber and a sufficient photopolymerization reaction does not occur, so that the curing property deteriorates.
In addition, although a hindered amine light stabilizer can be added in order to prevent yellowing, that is in the state of coexisting the hindered amine light stabilizer which suppresses a photopolymerization reaction (namely, traps radicals), so that the curing property deteriorates in curing by a radical polymerization reaction with a photopolymerization initiator.
That is, there is not yet a film having a urethane based polymer which is obtained by sufficient light-curing in the state of being added both an ultraviolet absorber and a light stabilizer, and has weathering resistance (yellowing resistance).